President Obama on Tuesday unveiled a comprehensive blueprint to combat rising seas and more frequent severe weather caused by climate change, including a long-awaited promise to regulate carbon dioxide as a pollutant at existing power plants.

The 21-page plan would expand production of solar and wind energy and includes billions of dollars in loan guarantees to develop cleaner fossil-fuel and other energy technologies. It also funds new efforts to armor communities against flooding, wildfires, and drought, and puts more emphasis on working with countries such as India and China to jointly lower emissions of gases that warm the atmosphere.

In a surprise, Obama also said the controversial Keystone pipeline that would bring oil extracted from Canada’s tar sands to Gulf Coast refineries should not be built if the overall result is more greenhouse gases.

“We don’t have time for a meeting of the flat Earth society,” Obama said to cheers near the end of a speech at Georgetown University. Multiple lines of scientific evidence point to emissions from power plants, cars, and industry as a major cause of the warming planet and changing climate.

“Sticking your head in the sand might make you feel safer, but it is not going to protect you from the coming storm,” Obama said.

The announcement builds on many of the president’s first-term initiatives and does not require approval of Congress, which has blocked previous efforts to pass legislation aimed at slowing global warming. However, the power plant rule, while short on specifics, is all but certain to face lawsuits, political opposition, and industry pressure.

“Whether the American people want it or not, he says he’ll do it by presidential fiat,” Senate minority leader Mitch McConnell, a Republican from coal-rich Kentucky, said in a speech on the Senate floor. “The message this sends should worry anyone who cares about constitutional self-government.”

Obama had directed the US Environmental Protection Agency to regulate carbon at new power plants, but it was not until Tuesday that he set in motion rules for existing power plants. Obama directed the EPA to draft rules within a year and to complete final rules within two. However, the plan lacked specifics about how significantly existing power plants would have to lower emissions.

The administration pledged four years ago to reduce carbon emissions 17 percent below 2005 levels by 2020, and about 80 percent lower by 2050, and Obama said his plan would help the nation reach those goals.

Efforts to trim emissions have been helped along by a new, steady source of domestic natural gas, extracted from underground shale deposits using a technology known as fracking. Natural gas emits fewer greenhouse gases than coal. The faltering economy has also contributed, by cutting emissions from manufacturing plants and other industry.

Obama also called for a renewed emphasis on adapting and preparing for climate change, including government support to make buildings and transportation systems better able to withstand flooding from rising oceans, a pressing issue brought into sharp relief by Superstorm Sandy last year. In Boston, where seas are rising faster than the global rate, officials have been working for several years to prepare the city for more floods, higher seas, and severe storms.

Reaction from environmentalists was largely positive while some industry officials described Obama’s plan as job-killing. Both sides, however, were circumspect and said they wanted more information, especially on how the power plant regulations would play out.

“As the president’s proposal recognizes, the nation must continue to rely on an ‘all of the above’ approach’’ to deal with climate change, said Thomas F. Farrell II, chairman, president, and chief executive of Dominion, which is in the process of selling Brayton Point in Somerset, New England’s largest coal-fired power plant. But he said the plan needs to give utilities regulatory certainty and flexibility to meet the reduction goals.

Obama’s plan aims to create enough wind and solar projects on public lands by 2020 to power more than 6 million homes, and reduce carbon pollution by a total of at least 3 billion tons through 2030 — equivalent to nearly one-half of the annual carbon pollution from the US energy sector — using appliance and federal building efficiency standards.

Carbon dioxide pollution is increasing globally, and is now around 400 parts per million; many climate scientists say levels should be no higher than 350 parts per million to avoid severe effects of rising temperatures and seas, and more severe weather.

Early in his first term, Obama attempted to take a leading international role in slowing global warming, but negotiations seeking global agreement on meaningful reductions failed in 2009 and were soon eclipsed by the recession.

It’s unclear how the new rules will play out in Massachusetts and the Northeast: The Bay State is part of a nine-state pact known as the Regional Greenhouse Gas Initiative, a market-based program that encourages dirty power plants to invest in cleaner technologies by having them buy increasingly expensive permits to pollute. Massachusetts also has strict air quality rules limiting mercury and other power plant pollutants, and those restrictions have made it more expensive for coal plants to operate. That, combined with the region’s increasing reliance on natural gas, has led to less electricity in New England generated from coal plants.

Some energy specialists say the new rules should encourage the US electric power system to rely even more on cleaner natural gas. Clean Air Task Force, a Boston-based research and advocacy group, found increasing use of gas and reducing use of coal could cut carbon pollution from existing power plants 25 percent below 2005 levels by 2020 with no increase in average household monthly electric bills.

“That’s because of two things — one, there is a lot of underutilized natural gas plants, and there is a lot of gas,” said Conrad Schneider, the task force’s advocacy director. “This is an opportunity.”

Internationally, the plan calls for the United States to work more closely with other major greenhouse gas-emitting countries on global warming. Secretary of State John Kerry, visiting India on Sunday, urged such cooperation. The plan also calls for the end of US financing of new coal-fired power plants overseas.

Obama’s comments on the Keystone pipeline, which is proposed to bring energy-intensive Canadian tar sand oil to US refineries, sparked speculation about whether he would ultimately approve the much-debated project. The State Department is still conducting an environmental review.

“Our national interest will be served only if this pipeline does not significantly exacerbate” climate change, Obama said, adding that the pipeline’s contribution to climate change was “critical” in determining the project’s fate.

Environmentalists said the president’s speech was encouraging, but did not make them completely confident the pipeline would be rejected.

“The nation’s top climate scientists are unanimous in describing the pipeline as a major threat to our planet,” Bill McKibben, founder of 350.org, said in a blog post on the climate advocacy group’s website. “But we don’t want to celebrate prematurely — the president did leave himself some wiggle room to approve Keystone.”